Another thing I'm worried about is that if I try for us to take a break or give myself some space, that it will send him into a tailspin.
Perhaps that is what is needed. Not just for you to heal, but for him to seek medical attention. Because for sure staying the SAME and you doing nothing different is not helping him. It is not helping you either.
We cannot help how we feel when we feel it. We do not choose it. We can only choose how we behave in response. REACT or ACT WITH INTENTION.
I am not familiar with ADHD. He def sounds depressed and self destructive.
I will point out you are not his wife or his mother. You are his GF, and you have every right to end if if this is not satisfying for you. Your whole first post is a long list of why this person is a HUGE drain for you and their unwillingess or inability to seek health help for themselves.
Do what you have always done? Get what you have always gotten. You do not like what you are getting. So... Someone must change their behavior to the situation. He is not changing his much.
It is you then.
Call his parents up perhaps? Tell them to get down here and do an intervention for their son?
Break up with him? Tell him to move out?
Ask him to commit self to hospital?
Determine the best course of action based on whatever your options are --- then ACT. Make a change.
Hang in there!